OK, first check that the socket you're plugging it into works. Also, try a different power lead for the UPS - the fuse in the plug may have blown or the lead itself may be faulty. (They're usually the standard IEC320 "kettle lead" type which is used for PCs so you should be able to borrow one from another piece of equipment.)
There may also be an internal fuse in the UPS; this is often accessible on the back either in a small pull-out "tray" under the inlet connector or as a small, round, black plastic cover about 1cm across which can be opened using a screwdriver. Check that this fuse hasn't blown too.
(If you've got a multimeter or something else that can be used as a continuity tester, that would be perfect for testing the fuses.)
Hope this helps,
Stephen