I have a garage in my backyard and I need some guidance.

Power used to work out in it then one day it stopped.  Not sure why, it happened when not in use.

There are two 20 amp breakers for the garage.  In the panel, one is marked lights and one is marked for receptacles.  The breaker for receptacles and is fine and will stay on.  The breaker for lights stays tripped and won't reset. I checked wires in the attic  and entry point to make sure there were no squirrels chewing on them and everything was good.  I replaced both breakers with new ones and same issue.  I've checked outlets for any loose wires.

Even though the breaker for receptacles is on, there's still no power to them.

Any other tips I can try or is it now time to call an electrician to trace the problem?

The breaker could be bad. If everything else checks out.

Bad ballast in a light fixture?

So to make a long story short...

the light circuit has a dead short.
the outlet circuit has no juice, but breaker stays on. (maybe a broken hot)

Either call an electrician, or pull the breakers and start tracing out your circuits (if you know what you are doing).

Of the garage is in the back yard, how does power get to it?  Is it detached and power is buried between house and garage?  If so, is it in conduit?  If not I'd guess aN underground critter damaged your feed.

Replaced breakers

Nope, newer replacements.

Yes, buried underground in conduit.

My next step is to check all of the outlets and switches again because the building is about 15-20 years ago, if that doesn't do it then I'll give up and call a pro.

How old is the wiring and what type of cable?

1. Never be sure of the breaker labeling. 87% of the time its wrong.
2. Turn off all the light switches and see if the breaker will reset.
3. If not, see where the conduit enters the garage and break all the joints loose at its first stop. If it still trips, its an issue in the ground. If it doesnt, then the problem is in the garage.

We've only been in this house for 2 years, so its been a fun process of exploring and fixing things.

So I found the junction box outside of the house that is right before the cable goes underground.  It is on the concrete patio behind the air conditioning unit and the cover only had one screw holding it on and it was slightly popped up full of ants.  No doubt water has been able to get in.   It definitely is an issue in this area and theres a few too many wires in a tight space for me.  So I'll be calling a pro tomorrow.  At least I narrowed it down a lot.

I would suggest having the electrician put a sub panel in the garage.
If you were to trip a breaker it could be reset without a trip to the house. Also, you could separate the receptacles ie: bench, walls, outside etc... Further it would give you a 240 volt feed if you were to run a welder or other equipment wired for 240.
I'm not and electrician but have a fair amount of time working on my own wiring (woodworking shop) I feel this would be a much safer approach.
Best luck with it and DO be careful..

edit for shitty spelling