I stopped playing piano at about the same stage as you (Grade 5) when I was a teenager and had lots of exam stress. I went back to it about ten years later without a teacher and it's one of the best things I've ever done. It was wonderful just to play again, but the freedom of being able to play whatever I liked and know I was doing it just for my own enjoyment completely changed my attitude to music. I now play more difficult pieces far more proficiently, compose my own music, and most importantly enjoy playing in a way I never did when I had a teacher breathing down my neck.
By contrast, my brother gave up the violin at about the same time as me, and similarly went back to it about a decade later. He went to a teacher, but the new teacher was so unpleasant that the experience put him off ever playing again.
On the other hand, if you're focused on making progress, particularly if you mean that in the sense of taking more grades, then a good teacher will almost certainly help, and if you have any worries about where to start or getting into bad habits then a few refresher lessons might not be a bad idea. I found that I was very rusty at first, but after a few weeks of playing everything just sort of clicked and my fingers remembered what they were doing.
The lack of a teacher has certainly led me into bad habits, particularly in the way I practice. I play what I like, and what I feel like, whenever I like, for as long (or as short) as I like, just to enjoy playing it and without a particular focus on improving my technique, which is a terrible attitude to have ;)
My general advice to most people in most situations is (quite hypocritically) to get a good teacher, but time and money are often in short supply, and sometimes good teachers aren't all that easy to find either. I wouldn't advise a beginner to try to learn without a teacher, but grade 5 is a reasonable level of proficiency, and with grade 5 theory you should be able to read just about any music and understand the basics of harmony. The most important thing is to get back to playing, and enjoying, music in whatever way fits in with your life now.
The best way to go about it depends very much on you. I started off by going back through my old exam pieces and then picking up music I liked in second hand shops. Finding music to play is so much easier now with so much of the classical repertoire available for free on imslp. You can try to approach things systematically and identify your strengths and weaknesses, or you can just play what you like. I worked my way through Czerny, but then I like etudes. The trickiest thing I found was identifying music that was sufficiently challenging to be rewarding without being too difficult and time-consuming to tackle. The biggest revelation I had was when I realised that the music I enjoyed playing wasn't the music I liked listening to.
Good luck, and do yourself a favour: ditch that digital for an acoustic the first chance you get!
