It'll create a backlog of cards to review, but that's surmountable. The number can become intimidating (I think my worst was around 2k cards), but there are a few ways to clear it. You can "just" get back into the daily habit, it'll take (by the default numbers, if one deck) # cards/100 days to clear out, more or less (some cards may come up for review again in that period so the actual number might be a bit higher). You can also up the daily review limit to clear it faster (I did this when I hit 2k, 200/day was about my personal time limit to spend on it so it took just over 10 days to clear).
In a simplified system suppose that we just double easy card review times, and reset hard cards back to 1. You have a card up for review today, you last saw it N days ago. Two scenarios:
1. You don't delay. You review the card now. It's either hard and reset to 1 (see it again tomorrow) or easy and you see it again in 2N days.
2. You delay. You review it M days from now. When you finally do it's either hard and reset to 1 (see it again the next day) or easy and you see it again in 2(N + M) days.
That's it. The algorithm has you covered if you delay. It doesn't do something silly like say "This card was supposed to be reviewed after 2 days, but you waited a month. You remembered it, but we're going to show it to you again in 4 days." The algorithms will take the delay into account (maybe not one-for-one) like I illustrated above.
