One of my clients does this all the time... wipes out custom code inserted in a text module. You'll have to ask if the designer/developer still has a copy of the code that was in there. I save the contents of all text modules locally before handing a site over... perhaps they were equally risk averse.
You really need to be careful because you're working on stuff that was presumably created and sold to your company for money as part of delivering the website. You really shouldn't be using the external company pages to learn how to use DNN. You need to ask for a special hidden page or set of pages to be created where you can play around with modules and the editor and not put the main pages at risk.
One thing you can do if you must work on the live content is export the contents of a text module (using the module menu if you have sufficient access), and if you mess it up, you can re-import that content back into it.
Another way is to switch the editor to Source view - if you see that button on the toolbar - and then copy and paste the contents into a separate notepad document beore making any changes.
But note: Some things like font sizes will have been set in the styles sheets for the entire website - not within each module. Whilst it is possible to go into any text module and force a different size on the text in there, it goes against the way the site will have been created. If there are fundamental font size issues, you should go back to the designer and ask for a broader change of style. If it is just a single item of text that you want to change then by all means go ahead and change it in the editor.
I'm pushing this point of not using the delivered and paid for website to learn how to use DNN because I have seen a portal completely destroyed in this manner by an employee who was very positive and enthusiastic but really had no idea.
Rob