You really, really, really need to update your Joomla installation! Can't stress that enough. The latest version is currently 1.5.26: http://www.joomla.org/download.html. (Note: at this moment you don't need to upgrade to the Joomla 2.5.x series, for now you can stick to the 1.5.x series.) The reason to upgrade is security, if your site gets hacked because you did not keep your Joomla installation up to date... and it is possible that you have a problem that is due to an older Joomla version, I have seen that before.

Update instructions are here: http://docs.joomla.org/Upgrading_1.5_from_an_existing_1.5x_version. It's rather easy, once you've done it once or twice.

You do need to make a backup before updating. See http://docs.joomla.org/Backup. I like the Joomla extension Akeeba for that (I don't have ties to them, I just like it). Based on their version compatibility chart (https://www.akeebabackup.com/compatibility.html), you need Akeeba 3.4.3 from this page: https://www.akeebabackup.com/download/akeeba-backup.html.

If you can, you might just want to create a subdomain on your website and duplicate your website to that subdomain using Akeeba. Then test your updates on that subdomain, where your users won't notice it and you can learn, test and try things. If you have the webspace for it, just spend some time on it.

It is quite possible that you don't need to update your extension (components, modules and plugins). However, periodically you do need to check if there are security updates for the extensions you use. And most of the time, updates are released that contain bug fixes and/or new features, so that might be nice as well.

After you have updated Joomla and still have problems:
- Enable Debug: Backend > Site > Global Configuration > System > Debug System = Yes
- Set Error Reporting to Maximum: Backend > Site > Global Configuration > Server > Error Reporting = Maximum
because a white/blank page is usually an error that is not showing and with these settings it will show. You need to post that error.

You also need to post your System Details: Backend > Help > System Info > System Info, for example:
- Database Version: e.g. 5.1.33
- PHP Version: e.g 5.2.9
- Web Server an version: e.g Apache/2.2.11 or ISS
- Web Server Operating System: Windows (e.g. the  (Win32) in the web server info
- Web Server to PHP interface:    apache2handler
- Joomla! Version: Joomla! 1.5.26 --> make sure this is the latest version: http://www.joomla.org/download.html
- User Agent: Mozilla/5.0 (Windows) - the browser and the operating system of the user