Samantha Smith. She was ranked in the 190-210 range her entire career, then reaches the 4th round of Wimbledon out of nowhere, beating then top 10 Conchita Martinez along the way, and then falls back to oblivion.
Katarina Studenikova. Ranked in the 50-70 range her entire career, out of nowhere beats then #2 Monica Seles at Wimby, reaches the 4th round and then falls back to oblivion.
Meredith McGrath. See Katarina, only she got to the semifinals that year, hyperextended her knee before her match against Sanchez Vicario and completely vanished after that. She played something like 4 matches in the next 2 years before quitting.
Clarisa Fernandez is a good example as well with her semifinal run at the French Open.
Lindsay Lee Waters getting to the quarterfinals of Indian Wells in 1996 and then only started winning tour level matches recently.