installed Monterey on iMac yesterday and now can't open computer. Keyboard dead.

Does the computer turn on ? If your Mac doesn't turn on

Keyboard issue - is this a Bluetooth Apple Keyboard or are we talking about a laptop computer ?

Yup - could have been the issue. Let it fully charge.

To compare - like yourself - turn on the KB and end of usage each day but do once in a while check the condition before switching off - just make sure there are not issues in AM. That, or keep the USB-C - Lighting cable handy for such a situation.

Apple Keyboard - connect the USB-C to Lighting cable directly to computer. This will enable using the keyboard long enough to login.

Keyboard - especially the newer KB from Apple - the battery may have gotten to low in power to function.

Check in System Preferences >> Keyboard and check how much power the battery in KB has

Welcome.

Do come back to ASC Forums when or / if additional questions arise.