The maximum hit points of a character is only the upper limit of the current hit points of the creature:
The current hit points of a creature (usually only called hit points) can be any number from the maximum hit points of the creature to 0.
This is reinforced in the healing section:
When a creature receives healing of any kind, the hit points recovered are added to its current hit points. A creature's hit points can not exceed its maximum hit point., so any hit point recovered in excess of this number is lost. For example, a druid gives a ranger 8 healing hit points. If the ranger has 14 current hit points and has a maximum of 20, the ranger retrieves 6 hit points from the druid, not 8.
Things that increase your maximum hit points (or eliminate an effect that reduced your maximum hit points) have no effect on your current hit points unless otherwise indicated.
This is reinforced by the wording of the help spell out:
Each objective maximum point of hit Y Current hit points Increase by 5 for the duration.
Note that help Increase your maximum HP and your current HP separately. If increasing your maximum HP would also automatically increase your current HP, it would be redundant and potentially confusing to establish both parts separately. However, because your maximum HP is only an upper limit, and its increase does not automatically increase the current HP, it is necessary to indicate in the description of the spell that both increase.
In summary: the maximum HP increase does not change your current HP unless otherwise stated (such as helpThe description of it does).
(Note that the maximum hit point decreases can reduce your current hit points, because your current HP can never be above your maximum HP.)