That happens to me all the time. Try restarting AE, that usually helps for me. There seems to be some kind of a bug in AE's handling of the operating systems' clipboard.
As Andy said, restart AE. For some reason on some systems the clipboard gets "polluted" by external influences and AE loses track of it. The cause is not really clear, so this is the best you can do for the time being. It may however be worth investigating otehr active processes and services and shut down the ones that are not really needed. That may help.
WOW guys!!! What a difference a day makes!! I'm running Vista Ultimate 64. I didn't describe the first symptom. When I finally got the data from Mocha and copied to the clipboard, I went to AE and Paste was greyed out!! I did a lot of things to try and get AE to see something in the clipboard. I think I finally clicked the Clear. Then I got the "nonresponsive" Paste.
Not getting an answer back from here for awhile, I shutdown (not your problem, my frustration). So, after getting you guy's responses, I rebooted the system this morning. Before I did anything else, I brought up the text file and copied and pasted the data to the clipboard. Brought up AE, clicked the comp needed, noticed Paste was NOT greyed out, did paste into the comp, and behold.... it worked wonderfully. A little adjustment of the comp (Mocha did more for position, scale and rotation than I expected) I was in fat city.
Thanks a bunch guys. This must be what happened at the demo of CS4 several moons ago at Adobe when they were trying to show Mocha off, they couldn't paste into AE.