I regularly like to download the community spotlighted maps from this site (which is an excellent one, by the way.) I had a great big mass download of maps this morning (around 20) to prepare so I could play them later. However, when I try to play Portal 2 it instantly crashes. Have I managed to delete an important file by mistake while downloading? What can I do to fix this? Is it the maps or something else that is causing the crash? I have tried rebooting.

Please help!

RU.

Right click Portal 2 in Steam

Goto to the Local Files tab

Click "verify integrity of game cache"

If you've managed to delete a file, or a file has gotten corrupt, doing this will repair it
If not, that means it's something else

It says that everything's fine with the files...
Any other suggestions?

Somebody has a similar problem on the Steam forums and they were told to reinistall. Is there anything less drastic I could do?

Did you remove everything you had installed beforehand? Else try figuring what files you might have overwritten, possibly in \cfg or \scripts. Or directly kill those folders and have the verify integrity cache thing replace them.

I did the delete local data thing, and now I'm really confused.

It said it was going to reinstall, with an installation time and everything, then seconds later it stopped, with the suggestion that the game was installed (it was no longer greyed out), yet when I clicked on it it said "Unable to sync with the cloud" and download-y type percentages appeared next to the game (suggesting it wanted to download an update).

I also noticed that a series of "portal2_assert_201107" files have appeared under "Steamapps-Common-Portal 2". And then, when looking for a solution, somewhere else suggested that the Portal programme should be there under that folder, but it isn't!

I am really, really, out of my depth and have no idea what I am doing.

RU.

Wipe out the game folder, then download it all anew from Steam. Let the pc handle it overnight, it might take some time and stop once in a while as you've seen.