Multiple documents open, but only seeing one
 

I came into work today and opened 5 excel files. Normally I see the 5 in the
corner and can use the alt; tab to go between files. When I opened them
today I only get one file and can only switch between files by clicking on
the window menu selection at the top. I tried to ungroup the files on the
task bar, but get the same issue. Does anyone know how to fix this? Thank
for any help or suggestions.