If no configuration options have changed it could be a PHP timeout. Possible causes might be complex page or one requiring much data or data from other sources... anything that could cause a timeout option limit to be exceeded.
You might be able to increase limits if you have appropriate server access (or a cooperative host). It might be posssible to tune the Moodle PHP page. Do you know which page you saw the error on? Had you added any modules or significant amounts of data or anything that tries to connect to other data sources shortly before you saw the problem?
General help
You have chosen to open "index.php" ...What?
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