Without having more information than
We were just working on the site a few days ago and everything was fine.
I would suggest reviewing your web server's error log. Your hosting company can often help you with this - and the related troubleshooting.
Something to try:
You may not have enough PHP memory going to each of the sites. To resolve:
1. If you have an access of PHP.ini file of the account/server then change the line in PHP.ini where line shows 32M to 64M:
memory_limit = 64M; Maximum amount of memory a script may consume (64MB)
2. If you do not have access of PHP.ini file then add this to an .htaccess file of your domain:
php_value memory_limit 64M
3. Try to add this line to your wp-config.php file to increase allocated memory to PHP: