Failed to validate files...what do you do now?

I ran the Validate file from steam on my TS2015 and got a message that says,
4133 files failed to validate and will be reacquired.
I hit the OK button but nothing happened.
I then restarted TS2015 and then closed it and went to run the Validate again.
Same message popped up when completed.
What am I suppose to do to reacquire the files? How do I know which files are required or affected?
What do I do?
4133 files failed to validate and will be reacquired.
I hit the OK button but nothing happened.
I then restarted TS2015 and then closed it and went to run the Validate again.
Same message popped up when completed.
What am I suppose to do to reacquire the files? How do I know which files are required or affected?
What do I do?