Each case is different though. Give it a few more days. 300 - > 3 makes no sense. Did your position drop way down? Can you see your ad now? How many impression?
Unfortunately you cannot undelete an ad. It is also unforunate that you cannot pause ads within an adgroup. Would be much better for test IMO.
To recover from your immediate problem, make a new ad with the old wording and let both ads circulate. It will take a few days for the CTR history to be reestablished, but after that, whichever ad performs the best will get the most weight in the rotation and you should be back to where you were before, or possibly better.
1. Google has been VERY slow at approving ads lately, so if you make a change on an ad w/o a backup, your ad may not be active for 3-4 days at times.
2. If your new ad is a bomb, you can simply keep changing it around until the CTR is better than your initial ad.
A tip - Regarding #2, when your new ad takes over your first ad, create ANOTHER ad and try to take over the new ad. If you keep doing this, you'll keep increasing your ad ctr and quality.