What's the word/verb meaning "to bend something inwards"?

Whether this is the lost word in your head I cannot say, but in some circumstances you might use scrunch.

Merriam Webster (M-W)

Scrunch:

to draw or squeeze together tightly; crumple — often used with up; to cause (something) to draw together —usually used with up

Cambridge

to make something or yourself smaller to fit into a small space

Collins discusses a little more than M-W and Cambridge. Here is an extract:

Collins

If you scrunch something, you squeeze it or bend it so that it is no longer in its natural shape and is often crushed.

Her father scrunched his nose. [VERB noun]

Her mother was sitting bolt upright, scrunching her white cotton gloves into a ball. [VERB noun + into]

to concertina:

OED

1. transitive. To cause (a thing) to fold, collapse, or wrinkle in a manner suggestive of a concertina's bellows. Also figurative.

2001 P. H. Jackson Chameleon Candidate i. 1 The force of the crash had concertinaed the vehicle.

2. intransitive. To fold, collapse, or wrinkle in the manner of a concertina's bellows. Frequently with prepositional phrase.

1998 C. Barker Galilee vi. 346 It [sc. a Mercedes] had concertinaed against the rear of the truck and was virtually unrecognizable.