I have a 10 yr old Maytag side-by-side. Came home the other night and the Freezer was at 40F. What is the most likely problem

To start: make sure refrig is turned on and running.
1.do you hear the fan blowing inside the frzr compartment?
2.pull refrig out and listen for compressor running. is compressor running/humming away or are you hearing a humming sound followed by a clicking sound?if you hearing a clicking sound than mostly
is your relay on the compressor that went out. look for a smal device on the side of compressor w/two or three wires. part# 12002794 $32.00

also make sure there's no frost build on frzr rear wall.that's another problem

The most likely components than can cause that are
the
freezer fan
defrost thermostat
defrost heating element.
defrost timer or control board (if have one)
if you remove the panel on the freezer side you will see the evaporator become a block of ice and snow.
this will confirm is the defrost components.
and check to see if the freezer fan works
(note on some refrigerators the freezer fan don't work whit the door open son manually press the door switch)