This error means that PHP (i.e. the program that runs liveSite) has run out of memory. If you are not trying to do something that requires a large amount of memory (i.e. generate large reports, export a lot of data), then we recommend reversing recent updates that you might have made, which might have created problems. For example, we recommend making sure that you did not add any secondary system regions for a Form Item View Page to any Page Styles (use a Form List View Page instead).
If the error is preventing you from being able to login in order to reverse recent updates, then we recommend using the link below (replace italics with your information).
If you are trying to do something that might require a lot of memory then we recommend increasing the memory_limit setting in PHP. If you do not know how to do that or you do not have access to do that, then we recommend that you contact your server administrator.
Also, if you are trying to generate an order or visitor report, then we recommend that you try reducing the date range, so there is less data for the report to analyze.