Please help,

Excel won't open, it just dissapeared, when I click on the icon anywhere in
the computer, it says that it's not installed.
When I check out the properties, it says it was modified recently, but not
to my knowledge.
Powerpoint already disapeared some time ago and I didn't pay too much
notice, I just simply downloaded the powerpoint viewer and made do, but the
excel is important for my work, what can I do?

Thankyou in advance,
Evan