DD went through this not long ago. though she was still happy to have her day nap, as soon as we went to put her to bed at night, she would start SCREAMING and crying and chucking huge tantrums and refusing to sleep. I had no idea what caused it, and still dont! I basically just let her fall asleep in our bed with us, and then transferred her to her bed or we would put her in her cot, and sit next to it so she could see us, then try and calm her and hold her hand/stroke her/pat her/etc. until she calmed down.
for a few nights we were lying down next to her cot while she fell asleep.
I think all in all it lasted about a week? I've never been so tired and stressed in my life! so i can sympethize