Mods suddenly stopped working.

[Bearer of the Girth]

So, I loaded up one of the mods I have installed on my PC. I played it before, but when I tried today, I got a white screen and then it crashed.

[Prinzessin Emily]

This is a problem, apperaing because of this Steam-pipe thing. (Not the best Idea Valve -_-)
Go in your sourcemods folder and open the folder of your mod.
There must be a file called "gameinfo.txt" open it in your favorite text-editor.
Than you have some parts of text. Search the Part of the file were just stand "FileSystem"
Replace it and everything that is following in the file with these:

FileSystem
{
SteamAppId 400

SearchPaths
{
game+mod |gameinfo_path|.
platform |gameinfo_path|.

// We search VPK files before ordinary folders, because most files will be found in
// VPK and we can avoid making thousands of file system calls to attempt to open files
// in folders where they don't exist. (Searching a VPK is much faster than making an operating
// system call.)
game_lv portal/portal_lv.vpk
game+mod portal/portal_english.vpk
game+mod portal/portal_pak.vpk
game |all_source_engine_paths|hl2/hl2_textures.vpk
game |all_source_engine_paths|hl2/hl2_sound_vo_english.vpk
game |all_source_engine_paths|hl2/hl2_sound_misc.vpk
game |all_source_engine_paths|hl2/hl2_misc.vpk
platform |all_source_engine_paths|platform/platform_misc.vpk

// Now search loose files. We'll set the directory containing the gameinfo.txt file
// as the first "mod" search path (after any user customizations). This is also the one
// that's used when writing to the "mod" path.
mod+mod_write+default_write_path |gameinfo_path|.

// Add the mod directory as a game search path. This is also where where writes
// to the "game" path go.
game+game_write |gameinfo_path|.

// Where the game's binaries are
gamebin portal/bin

// Last, mount in shared Portal loose files
game |all_source_engine_paths|portal

// Last, mount in shared HL2 loose files
game |all_source_engine_paths|hl2
platform |all_source_engine_paths|platform

}
}
}

For example here the Gameinfo.txt of the Mod Portal: Prelude:

Before:

"GameInfo"
{
game "Portal: Prelude"
title "Portal: Prelude"
type singleplayer_only

nodifficulty 1
hasportals 1
nocrosshair 1
nomodels 1

developer "Nicolas 'NykO18' Grevet"
developer_url "http://www.portalprelude.com"
manual "http://www.portalprelude.com/forum"
icon "resource/prelude"

FileSystem
{
SteamAppId 400
ToolsAppId 211

SearchPaths
{
Game |gameinfo_path|.
Game portal
Game hl2
}
}
}

After:

"GameInfo"
{
game "Portal: Prelude"
title "Portal: Prelude"
type singleplayer_only

nodifficulty 1
hasportals 1
nocrosshair 1
nomodels 1

developer "Nicolas 'NykO18' Grevet"
developer_url "http://www.portalprelude.com"
manual "http://www.portalprelude.com/forum"
icon "resource/prelude"

FileSystem
{
SteamAppId 400

SearchPaths
{
game+mod |gameinfo_path|.
platform |gameinfo_path|.

// We search VPK files before ordinary folders, because most files will be found in
// VPK and we can avoid making thousands of file system calls to attempt to open files
// in folders where they don't exist. (Searching a VPK is much faster than making an operating
// system call.)
game_lv portal/portal_lv.vpk
game+mod portal/portal_english.vpk
game+mod portal/portal_pak.vpk
game |all_source_engine_paths|hl2/hl2_textures.vpk
game |all_source_engine_paths|hl2/hl2_sound_vo_english.vpk
game |all_source_engine_paths|hl2/hl2_sound_misc.vpk
game |all_source_engine_paths|hl2/hl2_misc.vpk
platform |all_source_engine_paths|platform/platform_misc.vpk

// Now search loose files. We'll set the directory containing the gameinfo.txt file
// as the first "mod" search path (after any user customizations). This is also the one
// that's used when writing to the "mod" path.
mod+mod_write+default_write_path |gameinfo_path|.

// Add the mod directory as a game search path. This is also where where writes
// to the "game" path go.
game+game_write |gameinfo_path|.

// Where the game's binaries are
gamebin portal/bin

// Last, mount in shared Portal loose files
game |all_source_engine_paths|portal

// Last, mount in shared HL2 loose files
game |all_source_engine_paths|hl2
platform |all_source_engine_paths|platform

}
}
}

Now start the Mod, and it should work. (It even works without restarting steam ;) )
If it does not, than I don't know whats your problem, that was the way how I fixed my problem^^

[Bearer of the Girth]

I had actually found the answer on ThinkingWithPortals. Mods are functioning again.