A problem with my website, Please help

Hi,
Just one week back i am getting this error message.
Warning: unlink(/home/moto4ara/public_html/system/cache/cache.category.0.2.0.1310794481) [function.unlink]: Permission denied in /home/moto4ara/public_html/system/library/cache.php on line 14
Warning: fopen(/home/moto4ara/public_html/system/logs/error.txt) [function.fopen]: failed to open stream: Permission denied in /home/moto4ara/public_html/system/library/log.php on line 12

Warning: fwrite(): supplied argument is not a valid stream resource in /home/moto4ara/public_html/system/library/log.php on line 14

Warning: fclose(): supplied argument is not a valid stream resource in /home/moto4ara/public_html/system/library/log.php on line 16

My Website is www.moto4arabs.com
Please help.
Thanks,

what is the contents of your php.ini file in your root and admin directory?

Check the permissions on your cache directory. Start with 777 and the work your way to a more secure setting.
If 777 doesn't work then the above suggestion is going to be needed.

Maybe you hosting service tighten up permissions due to the recent hacking spree.