How do you open hood on suzuki reno when the latch won't work

Take a flat head and pry yhe latch either in middle to the left or push it down

if the car is push button electric and the battery is down, you will need to connect a jump start and then try, otherwise the cable must have snapped and it doesn work.
or just go to the local yard like Car Wreckers Brisbane https://www.qldcarwreckers.com.au/

probably find that the bonnet release cable is kinked and jammed in the open position or the return spring on the catch has fallen off and not pulling the cable back

did you ask SUZUKI if there is more than one cable.
some cars have 3 for different body types.
you buy this at suzuki? if not this is common.

how cold is really cold.
50f
-20F? what?

(the key is dead? or the remote lever?) why not say>?
i know of only these tricks. (all cars not yours)
1: crawl true rear set , hole,behind seat.
2: if electric latch, hot wire it.
3: if cable latch. find a way to get to cable and yank the casing
hard, while person 2 tries to open the trunk.
4: drill out the key hole, then usiing a screw driver open the trunk.
5: drive to any lock shop smith. and ask em. its trivial.
A; they (smith) have working keys, or can make keys or can pick the lock
or failing all that do #4.

the key is lost?
or the key dont work?
the remote lever dead too?
just a hint here.??????????

Raise hood,have some one go pull hood latch and you look at latch and see if it releases,if it doesn't then cable may need replacing,I would'nt completely shut hood cause you may not get back open,also check hood latches bolts and latch for something bent or broken.

try some wd40 and use a screw driver and work the latch might just be gunked up

Your engine's computer has detected a problem with it's engine management controls. The work you had done should have nothing to do with it.
Until you have the computer scanned (with an OBD2 scanner-Autozone will do it for free), you won't know what the check engine light is for. The scan will provide trouble codes for a technician to fix the problem. If it's a simple problem, you may be able to do it yourself. But only the code or codes will tell you why the light is on.
Good luck.