ISAs dont pay the opening interest rate for ever.
They can be a 1 year isa, 2 year isa, 3 year isa etc.
After the specified period of time the rate drops to a very low rate (which has what has happened to you).
DONT take the money out the ISA or you lose your tax free allowance.
Try to find a bank that will let you open a new isa but will let you transfer money from an old isa (it has to go from isa to isa to keep the tax free status)
Your existing bank may let you transfer to a new isa within the same bank.
If not go to another bank and ask if they will let you transfer money from another companies isa into their isa (some will some wont).
Last year I transferred money from a badly performing nat west isa to a Santander isa. Santander handled all the paperwork and it went from isa to isa.
You should check your interest rate EVErY year on every ISA you have.