Is the battle page saying your pet is unable to battle? Here's what you should do:
~Step One: Go to this page and look at your pet's health. Make sure it says 123/123 and not 0/123. You can heal your pet by going to the Healer.
~Step Two: If your pet is in healthy, check their hunger/happiness. If they are famished or unhappy, they go on strike and won't battle. You can feed them food or stick them in the resort for hunger, and play with toys or just give them a minion to keep them happy.
~Step Three: If your pet is healthy and full, then check and see if they're already in a battle. It's very easy to walk away from a battle and forget it, I've done it before myself. You can check that here, under the Statistic's Page.
~Finally, if you just adopted your pet, there is a 30 day waiting period before you can battle with it. That's a preventive measure to keep people from lending out pets for events.Unfortunately, if there's a plot going on, that means your pet is probably going to have to sit this one out.
~If all else fails, if your pet is healthy, full, and not stuck in a battle, then I would suggest Filing a Ticket. The UA's are super nice, and can figure out where your pet is stuck.
Some side notes on things that I've noticed come up a lot:
If you use Maleria's Cursed Glove in battle, it has a chance of making your other pets (the ones not in battle) hungry/depressed or knock them down to 0hp. During Fireside, the Bonfire can also knock your pet's hp down to nothing as well. Thus, people can walk around with injured pets and not even realize it until they try to battle.
Keeping your pet in the resort/inn has no effect on their battling. Contrary to rumors I've seen floating around, you can still most definitely battle with them. The only thing the resort does is feed your pet; in fact, a lot of battlers use it so they don't have to keep shoving food down their battle pet's face.

wow you have been very busy at getting helpful trends :)
Live your Life to the fullest cause you never know what will happen tomorrow......
Pft, this one's just a copy/paste from the board I made for a few events ago. It never fails, though, there's always a sudden surge of people unable to battle, so I just keep reposting it. I'm just late in getting it up this time.

Thanks, I didn't realize there was a 30 day waiting period. I started playing a couple days ago and saw the news about the cruises but guess I won't be able to do it. Good to know! :)
Thanks this helped me a lot! Didn't realize I was still in a battle....[facepalm].
Thank you for the help! If your pet is in the resort, is it still able to battle? I'm guessing the resort does nothing but keep your pet at full Hunger.
[img align=right]http://i46.tinypic.com/2lu713k.png[/img] Thanks to
Yup, you're right. All the resort does is keep your pet fed, and its the most convenient way to do so. I think the vast majority of battlers keep their pets in the resort.

Thank you so much. And thank you for these amazing threads! They really are helpful. On a side note, I ADORE your HA.
[img align=right]http://i46.tinypic.com/2lu713k.png[/img] Thanks to
Thank you! I didn't realise my pet was alredy in a battle. XD lol
For anyone battling the new challengers and running into problems, check out this thread.
Thanks for this, it looks like I have to file a ticket, I can't figure out for the life of me why my pet is stuck... And it's only my battle pet, so strange..
[tot=Halibel] [egg=Halibel] [tp=Halibel]
If Niix is your battle pet, it looks like they have 0 HP, so that's probably it.
Omg, are you kidding me LOL, thank you!! Sigh, I feel a little dumb now, hahaha......
[tot=Halibel] [egg=Halibel] [tp=Halibel]
Do you know how long I have to wait between putting my pet in the resort and him being fully fed? I already fed him up once today, I don't really want to spend a buttload of Lounge Trinkets until he can battle again.
they/them/theirs, please.
I think it's an hour after you put them in, but I'm not sure.