How to open a file cabinet with no key?

If there is no key at all for it, you'll have to drill it out. Drill the center all the way to the back of the lock. You might still have to "strong hand" the lock a little to open it but you will then be able to use a screwdriver to unlock it. You'll be able to buy a new lock if you wish from a locksmith.

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Without breaking the lock? Try turning the cabinet upside down and see if the locking arm falls out of position; had it work on a cheap lock.

The two previous posters hit it right on. I have drilled out more than one locked cabinet. It will get it open, but the lock will be forever useless.

A crowbar and a sledge hammer should do the trick.

Try a wire coat hanger or anything that is sharp/pointy

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Bye Bye

just take the lock out... use a hammer and screw driver

I have a set of picks that I'm pretty good with.

I'll do it for you.