A Disaster Happened
Tapes are physically robust, there may be solutions. Don’t panic and seek professional advice immediately.
Fire, Flood, or other natural disaster
The Association of Moving Image Archivists has put together useful links on disaster recovery.
There is good news that many tapes CAN be saved if you act quickly. However, you need to go to the Specs Bros. Web site. They have emergency links on their front page.
I erased or recorded over a tape
I am sorry to report that in almost all cases this is impossible to recover. There are the few exceptions, but none of the experts that I have talked with hold much hope. The only way a tape can be recovered is if the machine erasing it did not live up to its design specifications which were to completely erase the tape.
In some data applications, erased data can be recovered, but in analog recording the most one can hope for is a former shadow of the original recording. If you can get anything, it might be enough to transcribe the text. This, however, is theoretical, and I’ve never seen a case where anything was recovered from an erased analog audio tape (cassette or reel).
To put this in perspective, the government still has found no one who can recover the 18-1/2 minute gap in the Nixon Watergate tape. They ran a test in 2002-2003 with tapes similar to the Watergate tape and recorded and erased on the same or similar machines. They decided that no one showed enough promise during these trials to warrant letting them try the real thing.
The best protection against erasing a tape is, on a cassette, to break out the record protection tabs on the back edge of the cassette. You would be surprised the number of cassette tapes I receive for transfer that do not have the tabs broken out.