Ok - please be gentle, I am very new to WP and don't have any technical knowledge the content of many of the threads I've searched here.
I think my WP did an upgrade recently. Since that point, I've been unable to log onto the admin screen. When I enter the link, I just get a white screen. I understand that this is common with php issues, following upgrades?

Not sure if this is related, but I have found an error log file on the system. I am not sure if this will throw any light onto the matter:
[31-Oct-2011 16:25:47] PHP Fatal error: Allowed memory size of 262144 bytes exhausted (tried to allocate 122880 bytes) in /home/scubatec/public_html/scuba_blog/wp-admin/includes/dashboard.php on line 550
[02-Nov-2011 07:12:56] PHP Fatal error: Allowed memory size of 262144 bytes exhausted (tried to allocate 122880 bytes) in /home/scubatec/public_html/scuba_blog/wp-admin/admin-header.php on line 118

How can I fix this?
I've tried to follow instructions on the WP forum to rename my plug-in folder (and create a fresh/empty one), but that doesn't seem to do anything. It should (?) have resolved any issues caused by plug-ins after a (failed?/successful?) update of WP?

Here's what I read:

1) Either rename/remove Plugins folder via FTP or disables all the Plugins from database.
Tried that. I found 2 plugins folders. One under public_html and one under www. I really don't know what I'm working with here?!

2. 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 256M:
memory_limit = 256M; Maximum amount of memory a script may consume (64MB)

Got no access to the PHP.ini file?!?

3. If you do not have access of PHP.ini file then add this to an .htaccess file of your domain:
php_value memory_limit 256M

See above.

4. Try to add this line to your wp-config.php file to increase allocated memory to PHP:
define(’WP_MEMORY_LIMIT’, ‘256M’);

I don't know where to find that. Very cautious to mess with anything that has site-wide implications (especially because I am not 100% sure how to back-up and restore if it goes wrong).

Other than that, I am stumped. I don't know much about database management - struggling even to know how to back-up. Worried terribly about losing all my forum posts (12 months of hard work).

Can someone please take the time to talk me through this and help me regain access to the admin screen so that I can breathe life back into my (now very silent) blog? Please assume I am the dumbest noobie you met - and have no idea what I am doing.

Thanks in advance.