There is a now closed question that I have noticed from the VTC queue. The problem was that 2 of 3 players are going to miss a few sessions, but one player wants to continue. The author asks about a way to balance an encounter for just one adventurer who stays. This is indeed a very broad subject, partially because the OP didn't even include any details about this potential lone adventurer aside from being level 5.
I have also never played D&D 5e and don't plan to do it in the near future, so I would not be able to give any kind of advice on balancing a solo adventure in D&D 5e.
However, I think that the question suffers from an XY problem: it asks about an attempted solution (balancing a solo adventure), while a correct solution is not to continue the campaign while the majority of the group is absent.
I have experienced similar situations myself, my friends have experienced them too, and a tested solution was that this particular campaign needs to be suspended while someone cannot attend a session or two, which I have given as an answer.
My opinion is that this knowledge is very useful, 5 people who have upvoted my answer likely agree. However, the question itself is indeed too broad and close-worthy.
What is also interesting: the original title in the first revision actually lists the problem as 2 of 3 players being temporarily out, not as having to balance a solo encounter.
What, if anything, can and should be done in this case?
