We all have on and off days, and as a designer, I can say that this is especially applicable to my design work. Somedays I feel creative and I can think of excellent ideas and can execute those ideas, whereas other days my mind goes blank and I just can’t think of anything good. That can be so frustrating!!
In order to get some ideas and get those creative juices flowing you can do many things. First off don’t forget what you learned in school. You need to find a process of design that works for you and don’t just start designing without a plan. Seriously think about steering away from the computer and go back to pen and paper and sketch out some ideas. Draw thumbnails and keep drawing them, you will be surprised at the things you start to think of. You can then combine/add/subtract ideas until you have a solid idea that works. Remember that you can sketch rough ideas out faster and more easily than if you tried to do it on the computer (for the most part).
Other ways you get thought going is changing your mood or your environment around you. Whether it be moving to a new location to work or putting on some headphones and listening to some tunes, it can spark your brain for creativity.
An almost foolproof way of finding inspiration is by going to design sites. Design Meltdown and Fuel Your Creativity are my favorites, and from there you can find links to more sites that can help spark ideas as well.
This is not an exhaustive list obviously, just something to help get you started for when you get stuck!