1.
there is only; there is nothing but
Attached to a noun or a quantity, it states that nothing else exists and frames the amount as insufficient. 千円しかない means the thousand yen is all there is, and it is not enough.
2.
have no choice but to; the only option is to
Attached to a plain-form verb, it states that a single course of action remains. It carries resignation: the speaker is not choosing the action so much as running out of alternatives.