Message and email and write if you have an address - same content in each.
"I consider that as it is now x weeks since you promised to collect Ddog, that you have abandoned your dog.
If you do wish to collect your dog, please do so before x date (date of 7 days time, thats sufficient, less than that is likely unreasonable).
Failure to contact me or collect dog within x date means you agree to me taking ownership of Ddog.'
Keep all the paperwork you have, all receipts for food, vet bills, whatever - these will be used to back up your case should she take you to small claims to get the dog back.
If you have anything in writing where she asks you to look after the dog and says when she will collect the dog, keep hold of that too.
It isn't perfect - if she can prove theres a reason she could not contact you about the dog, she could win - but every text she hasn't answered, every issue with the dogs care she has not replied to, every bill that remains unpaid, all adds up to her relinquishing ownership of the dog to you.
If she does come looking for the dog and you want to keep the dog, let her know there will be a bill for the dogs upkeep for the last however many weeks, that should put her off if she's just doing it to spite you.If she has genuinely met with some disaster, she'll happily pay up what she owes and apologise profusely in order to get her dog back.