* Ad was mistakenly deleted
* Campaign or Ad Group was mistakenly paused
* Campaign end date was set by advertiser, and has now passed
* Keywords have been disabled
* Ad was reviewed and disapproved
* Max CPC or daily budget was changed, so that Max CPC is now higher than daily budget
I'm sure I'm missing some possibilities - so really, contacting AdWords support is your best bet - as they can look at the account and tell you very quickly what is happening.
AWA
By the way I tried to create an impresion by type some of my keywords in google and my ad appeared but this impression counter in my control panel did not refresh!
Just a quick clarification on your observation, above, Etarwin:
There is (roughly) a three hour delay between the time an impression or click happens, and the time that you'll see it in your account stats.
And then, just as an aside, if our system thinks there is something that suggests your impression or click may not be 'valid', it will be filtered out of your stats altogether. So, there is always at least a possibility that your impression may not appear at all.
AWA