I hope you have a backup. And keep your backups
safe and offline.
http://www.mvps.org/dmcritchie/excel/backup.htm
Check your font in the fontbox
Right click on the sheet tab, view code,
anything in there?
Tools, Autocorrect
check the righthand column for y's
Does this happen with just one worksheet, the entire
workbook, all workbooks, all MS Office programs
like Excel and MS Word. Does it happen on new
workbooks, new worksheets.
What do you see on the formula bar for such a cell.
What do you see for the formatting for such a cell.
Have you used your workbook since April 1st.
Anything strange in File, properties