Creo 4.0 Cannot Edit Sketch

Hello everybody.

I'm currently using the educational version of Creo 4.0. I saved my project as a .prt file, but when I open it again, I cannot seem to edit the sketch or even extrude on specific surfaces. 

It seems like I have to do a part all in one take, and I want to save a .prt for future edits. 

I just don't know what causing me to create a fully finished part in only one sitting. 

Thanks