how big is your database? Do you have enough space on the disk where you putting the backup file if not then you need to amend the location as :-

backup database yourdbname to disk = 'd:\yourdbname.bak'

**Assuming d drive is accessible and have enough space to put the backup file.

In addition, you have other option as well like :-

1) considering differential backup instead of full backup.

2) considering the third party tool like litespeed to take compressed backup

3) if you in 2008 or above, you can use sql compressed backup.

*************edit

Thanks for correcting me Gail. I thought database is accessible and the problem is with backup.