Currently, I'm running a site of mine on Nuke 7.3. If I wanted to upgrade to 7.4, will I lose any of the content I have on my site?

Nope, we upgrade this site.

1) Backup your site and database.
2) Upload the new version, answer yes to overwrite.
3) Edit the new config.php with the info from the one you backed up.
4) Upload the upgrade73-74.php to the root directory (where your mainfile.php is), and point your browser to it.
5) Delete the upgrade file.

You will have to redo any mods you've made to the forums, language files, etc.

Ok that sounds like it isn't too hard to do. Now, I have a huge problem and for the life of me I cannot figure out what is causing this. I'm wondering if its something having to do with the Nuke Sentinel but I'm not all that sure. Here's the problem:

I cannot log into my website from the normal users log in area under site info. ( http://rpg.sharnyl.net if it helps you at all) I can log into the admin area but not my user account, is what I'm saying. If I do try to log in to the user account, I get redirected to a page that says the Login Incorrect, please try again. But I know that the login isn't incorrect. Also, when redirected to that page, it pops up the security code to input. Only, there aren't any numbers to input. All I see is the words Security Code in the box where the numbers should appear.

I'm not sure if this makes any difference or not, but our server was down for about five minutes this morning. I had no troubles logging in to the user account or admin panel before the server went down. It seems that I'm the only one of the registered accounts at my site that is having this problem. Other members are able to login to their accounts just fine.

There are a few things that look as if they have changed in the admin panel though. the usergroups has an _ before it so it looks like _UGROUPS and the users control panel where they can edit their account information has the following in it now: _SUBUSERASK and _SUBPERIOD
I have no idea what those are or what they are for.

Recent blocks that have been added are:

XBOX LiveGamers Tags
Never Ending Story
Travel News
Affiliates
Sports Ticker
Symantec

Again, I don't know if any of these things could be the cause of my troubles but I figure it couldn't hurt to try to cover the bases of all possibilities.

_SUBUSERASK and _SUBPERIOD
Those are like that when you are missing the language file definitions for that particular block or module.

Look in your language file and check the install instructions for language definitions for the affected block.

Ah, thanks I managed to fix everything by upgrading to the 7.4. I appreciate your help, thank you again.