Problem with detecting an external monitor after force shut down

I used a Dell monitor at the office with a DP to USB-C cable and everything worked well until one day I forgot my Mac charger at hone, and after several hours of battery saving, the Mac shut down and afterwards could no longer detect the Dell monitor. I tried to turn both monitor and Mac off then on again; when I hold the Option key to view the "detect display" option in the display settings, I cannot actually choose this option. How can I solve this? Thanks