Locked out of iMac

I have bought a new iMac M1, which comes with a mouse but no Magic Trackpad. So I ordered one and in the meantime I paired my old trackpad to the new iMac. When the new Trackpad 2 came I removed the old one and connected the new one. All fine. Except that when I came to turn on my old iMac I couldn't, because I had disconnected the old trackpad. So I have locked myself out of the old iMac (late 2012) and can't find a way to get it. I can turn on the machine and get the login page - but how can I get logged in again, to reconnect my old trackpad? Help?