My Cat Is Having Trouble Moving Her Bowels, What Can I Do?

My 15 yr old cat keeps trying to go to the bathroom and she can't. She was in the litter box and nothing is coming out. We went outside and she tries to go in fromt of me as if she wants me to do something. I am feeling very helpless. Is there anything I can give her to help. There is no where I can take her until tomorrow and she needs help now.