If the file still exist, but just isn't usable. Then, it's been corrupted for whatever reason. I have this happen to me when a user shuts down the computer w/o properly exiting and at the time, I have an active memo file up.
It's not straight forward. But, I've always been able to repair it because VFP has a repair capability built in.
The problem is that it always takes me a while to get to work.
You have to delete the data table from the DBC first and make it an independent table. Then, if you try to do a use exclusive on it, it will tell you the memo file is bad and if you want to repair it.
Then, add it back into the DBC. However, if you had any long fieldnames in the DBF, you have to rename then since they're lost when you separate the table from the DBC.
That's why I use standard 10 char or less names. So, if I separate and re-add a table, I don't have to deal with the loss of long field names.