[Koha] Software error

Sir, I was on leave for two days n when i returned i m facing a new problem.WhenI give the required address in the address bar i get the following error message Can't call method "prepare" on an undefined value at /usr/local/koha/opac/cgi-bin/opac-main.pl line 18. So,now again my work is stuck. Thanking You Priyanka