When you find yourself out of your routine it can be hard to imagine getting back into it. This is especially true for exercise. Going for days or weeks without getting the exercise you're used to can lead to a pattern that cuts it out completely.
I can tell you from firsthand experience that the easiest way to get back into your routine is to just jump right back in. I was recently on vacation for a week and easily slipped from my normal exercise routine. Coming home, I was dreading starting back up at the gym. It's so easy to make excuses when you have so much to catch up on and so little motivation.
On a last minute decision I trekked over to the gym thinking I could at least do 30 minutes of cardio, if nothing else. By the time I got started I decided that since I had made it all the way there I might as well do a little more. By the end, I had incorporated an ab and strength training workout as well.
When you think you just can't do it, start small and see where that takes you. I promise that you will never leave the gym kicking yourself for going. It's almost always worth it.