If you are on the NT, do Ctrl-ALt-Del, ther e choose the taskmanager, then go ahead and kill the tnslsnr.exe dbsnmp.exe, vppdc.exe, jrew.exe, launch.exe and all the other relevent processes. then try starting the database as follows.

startup mount
then
alter database open;

If you are not successfull in that of shuting down, issue

shutdown immediate
or shutdown abort

If you had issued shutdown, it would wait untill the connections are commited and released by the user. So you would want to issue shutdown immediate It will take a few seconds to minute. Then if that goes through, issue startup
.

Other wise issue startup force

Hope this would help you, good luck
sam