Baby is fighting sleep?!!???? WHAT IS HAPPENING

My ten month old was a wonderful sleeper up until a few days ago?? She used to go right to sleep when I put her in the crib & sleep through the night but ever since a few days ago she has been fighting sleeping so hard. She acts tired but I'll put her in the crib and either she'll cry, just sit there awake, or fall asleep for a few minutes and wake up crying. The past few night I've continued to stick to the same routine and she takes an hour or so to finally fall asleep at night & that's with me going and rocking her a few times and finally giving up and and letting her cry it out. What is going on in going to lose my mind if she doesn't start sleeping again!!