A non-responsive remote is usually batteries or the receiver sensor in the indoor unit. In 80% of cases you can fix it yourself; if the receiver is dead you'll need a technician.
See if any of these signs match your device — each one increases the likelihood of this fault.
These are the causes we encounter while repairing AC remote not working — ranked from most to least common.
Weak batteries (even if the display still lights up)
IR emitter on the remote burnt out
IR receiver inside the indoor unit failed
Remote LCD damaged
Direct strong light (sunlight) on the receiver causes interference
Any of the following = a danger signal — call now and don't try to fix it yourself.
On-site diagnosis, clear pricing before any work, and a documented 6-month warranty.