Cannot move the arrow on the screen

Ensure the mouse is charged.

If this is an Apple Magic Mouse, the charging cable can usually also be used to pair or re-pair the mouse with the iMac.

Try a different mouse. Wired, if you have a USB-C mouse, or with a USB-A to USB-C adapter and a USB-A mouse.

Un-pair and re-pair the mouse, and see if that clears this. To access the controls and enable mouse keys or pair or un-pair a mouse, use Control-F2 to open the Apple Menu, then arrow keys and return and escape and tab to navigate menus and controls.

You can set up a shortcut—as mentioned in the link above—to toggle mouse keys on and off.

If the mouse has failed, acquire another.

If this is the mouse that came with the computer, it must be charged first as MrHoffman states.

And I am not trying to be flippant or condescending here but, as this is common cause of mouse and keyboard issues reported here, I must ask: Apple mice also have a power switch. Did you find that?

Do the keyboard keys 7, 8, and 9 move the cursor around?

If so: Control the pointer using Mouse Keys on Mac - Apple Support