USB drive won't open

Help! I've got a VERBATIM Secure Data USB device (4Gb I think) that I bought in 2012.
No problems till now- I can't get to the password entry screen - I just get a yellow triangle with "!" inside it, and an OK button to press. So I can't open my files.
Can anyone help me!
My back-up is a month old.
 
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