what do the icons in the corner of the surface uefi screen mean?

If it's just stuck on the Surface logo and won't go further, force it off first. Holding power for about 20 seconds does that on most models.

which icon exactly? the common ones on a Surface boot screen: a thermometer means it's too hot or too cold to start, so let it sit at room temp. a battery with a plug means the battery is too low to boot, leave it on the charger 15-20 min before trying again. a red bar across the top means Secure Boot is turned off in the UEFI settings, that one isn't a fault, just a warning. if it's something else, describe it and the model and someone will know.