Hi there!
Don't panic! BREATHE!!!
This has happened to me SO many times, and this is what I do:
1. Reasses what and how much you are eating. Are you tracking your intake? You really need to be consistent with respect to intake so that you give yourself the BEST chance of losing the weight. You can do this through a structured program like WW or through calorie counting, like TONS of the successful maintainers do (use Fitday or The Daily Plate or Sparkpeople). But this is the first step.
2. Do you chart or graph your weight? I use a free online program called "Fridgegraph". This lets me see how I do week to week, and I found that I have a weight loss cycle. I lose ALOT in week 1, a bit in weeks 2 and 3, nothing or am up in week 4, and then I repeat the cycle. So because I know this, I don't freak out (so much) when the scale doesn't budge. Because OVERALL, I am losing.
3. We are now in summer (pretty much!)! Are you drinking enough water? Has your salt intake been higher than usual this week? Have you upped your exercise? All of these will account for bumps up in weight. They are temporary, usually due to water retention, and will resolve themselves with time.
Let me put it this way: last week, I lost 3 lbs! Amazing for me!!! This week, I am UP 0.5 lbs! Eeek! I did everything right! So what happened?
I reassessed my diet. I went out for dinner twice (still within my eating plan, but I can't control the salt nor oil content directly). I assessed my lifestyle. I realized that I have doubled my exercise (I am doing a hot yoga challenge, with 30 classes in 30 days). I checked where I am on my weight loss cycle, and am on week 2. So, I don't lose alot in week 2 anyways. Result? I have done everything right, but my weight is UP, and I'm pretty sure that this is temporary.
I am still a bit irked, but not completely upset, which I WOULD have been in previous weight loss attempts.
Anyways, you're bored now, I'm sure! In short, hang in there, assess your diet, exercise, and lifestyle. Relax, and stick with it. You WILL see results -- they just won't be linear nor completely predictable nor super, super fast...

Kira