copy pasta from pony fortress 2 (i would have linked steam forums but its down for maintenance for the moment):

How to fix it:

Here's what I do whenever I have the problem. It works every time. Take note however, I have a MAC so it may be different.

1. Go to steam and right click TF2 or go to the little menu's at the top and go to TF2's properties on steam.

2. Once at the properties, go to the 'Local Files' tab.

3. Once there, click 'Verify Game Cache' and let it do it's thing. (If this itself is stuck at 100% for a couple minutes as well, just close it and proceed to the next step. ALSO, if it is stuck at 0% and nothing happens, this means nothing needs to be verified and you can just skip this step.)

4. Once done, click the button below it which is 'Defragment Cache Files'. Let it sit there and defragment the files. (If this is stuck at 100%, LEAVE IT. Sooner or later it will notify you when it's done.

5. Done! You should be able to start TF2 without having to validate the files again.

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try their suggestion and lmk if it fixes it

I've seen this problem quite I bit.

1. You could manually validate in case the automatic validation is missing something. Right click TF2 in library, click properties and under local files select "validate game cache".

2. Your hard drive may be skipping or damaged, open "Computer" (If on Win 7) right click on HDD and select properties, select tools and then check for problems.