How to get database out of single user

Hi

SQL Server 2005. Had changed database to single user/read only option. Now I cant change database back to multi user and all commands are getting stuck.

Msg 924, Level 14, State 1, Line 1
Database 'db1' is already open and can only have one user at a time.

I have tried all below and failed to get around this issue

sp_dboption db1, 'single', false

alter database db1 multi_user with rollback immediate

I cannot select the spid with as I get Msg 924 with below commands which all get stuck.

select spid from sysprocesses where dbid = (id of database)

sp_who does not show

What can be done to get out of this as this is a production server and difficult to restart midweek.