Are there any buses or trains running from Isaan cities to Bangkok at the moment or are they all closed?

I think all closed for since beginning of this month.

My friend came back just yesterday to bangkok with her family by private transport.

You can try to find of fb local groups some foreigners, who travel to bkk for vax or to the airport to drop their family/friends

How about using the train ?     I think it is a night train.
I use the train from Banphai to KK and back, lovely way to travel and all that fresh air coming in the open windows.     Train costs 9 baht for a 40 minute journey ( 2 stops  ), also you can walk to Central in 10 minutes.
Not sure if there are flights.

2 hours ago, Speedo1968 said:

Not sure if there are flights.

No commercial flights.

I haven't seen a bus for many weeks.

A granddaughter had to be brought back to her college near Korat from here.

Uncle and parents drove with pickup.

Almost 500 km round-trip during daytime.