We've spoken about how you are not finished yet with your project, how to avoid burning bridges in conversation, how to tell if a project is dead, or merely resting, and on knowing when to retire from a project. Today I'd like to talk about coming back to a project which you have taken a hiatus from.
Whether your break from a project was planned or unplanned, whether you were fundamental to a project's progress, or "just" a helper, your absence will have been felt by others associated with the project, even if they've not said anything. It's always a nice thing to let anyone who ought to know, know that you have returned. Depending on your level of integration into that project's community and the particulars of your absence, your own sense of privacy around any reasons, etc., it can be worth letting the community know a little of why you were away, and why that means that now you are back.
If your time away was unannounced, unplanned, abrupt, or otherwise disruptive to the community, it can also help to mend things if you are prepared to apologise for your absence. NOTE I am not saying you have to apologise or excuse the reasons why you were absent, merely to note to the community that you are sorry to have been away and that you look forward to reintegrating.
Just like I'd recommend when you join a community, make a point of discussing what you intend to achieve in the short term, in case there's someone who wants to assist you or has been working on something similar. Also make it clear if you've kept up passively with the community in your time away, or if you've essentially a blank for the period of your absence. That can help others to decide if they need to tell you about things which might be important, or otherwise relevant, to your interests and plans.
If you were specifically, and solely, responsible for certain things when you were previously part of the project, you should, ideally, go back and check if anyone has picked it up in your absence. If so, make a point of thanking them, and asking them if they wish to continue with the work without you, share the burden with you, or pass the work back to you entirely. If that responsibility was part of why you had a hiatus, ensure that you don't end up in a similar position again.
There are many more suggestions I might make, but I'll leave you with one final one. Don't expect the project you re-join to be the same project you left. In your absence things will have progressed with respect to the project, others in the community, and yourself. Don't be saddened by this, instead rejoice in the diversity of life and dig back in with gusto.