1. Write some description. Where is your viewpoint character now - what can they see, hear, touch? What are they wearing? Go into as much detail as you can. It doesn't have to fit into the plot, it's enough that you're writing, being able to use it at some point will be a bonus.
2. Don't get hung up on writing chronologically. If you fancy writing a scene near the end, do it and don't worry about the fact you haven't set it up properly. You can always go back and do that another time.
3. If you're finding a scene difficult to write, jump over it. Make a few notes - Jess buys a motorbike but her credit card is refused - and then carry on. The problem with the scene will almost certainly resolve itself later - perhaps Jess needs a car, not a motorbike.
4. Tell yourself you're just going to write for ten minutes about anything, and that's your quota. If you really can't think of anything to write about, start with the phrase 'I remember when...' and take it from there.
5. Stuck for a story line? Choose a fairy tale or nursery story. Re-write it with a contemporary feel or give it a twist - make the Big Bad Wolf the hero. Shakespeare was notorious for nicking plots from other writers and if it's good enough for Shakespeare...
Generally I think the best thing is to relax and accept that we all have off days and that creativity doesn't flow from some convenient tap that can be turned off and on at will. On the other hand, don't use this as an excuse for sidling out of writing all together as one day will slip into another and a month will go by without you writing a word. The best solution for writer's block is to develop a writing habit.