I tried logging onto my database through sqlplus and got the following error
ORA-01092: Oracle instance terminated. Disconnecttion forced

I therefore went to the dos prompt to connect to server manager to get it started but it did not. Below is what I did:

c:\oracle\ora81\bin>set oracle_sid=orcl
c:\oracle\ora81\bin>svrmgrl
SVRMGR>connect internal
SVRMGR> shutdown
ORA-03114: not connected to ORACLE
SVRMGR> startup
ORA-01031: insufficient privileges
SVRMGR> connect internal
Connected.
SVRMGR> startup
ORA-01081: cannot start already-running ORACLE - shut it down first
SVRMGR> connect internal
Connected.
SVRMGR> STARTUP FORCE
ORACLE instance started.
Total System Global Area 38322124 bytes
Fixed Size 65484 bytes
Variable Size 21405696 bytes
Database Buffers 16777216 bytes
Redo Buffers 73728 bytes
Database mounted.
ORA-03113: end-of-file on communication channel

NOTE:
I'm running small databases on this machine.
O/S= WINDOWS NT
ORACLE VERSION=8.1.5.0.0. oracle 8i Enterprise edition

ALert log shows-

Starting ORACLE instance (force)
License high water mark = 3
Fri Apr 20 15:21:34 2001
Instance terminated by USER, pid = 211
Starting up ORACLE RDBMS Version: 8.1.5.0.0.
System parameters with non-default values:
processes = 200
shared_pool_size = 15728640
java_pool_size = 20971520
control_files = D:\Oracle\oradata\orcl\control01.ctl, D:\Oracle\oradata\orcl\control02.ctl
db_block_buffers = 8192
db_block_size = 2048
compatible = 8.1.0
log_buffer = 32768
log_checkpoint_interval = 10000
log_checkpoint_timeout = 1800
db_files = 1024
db_file_multiblock_read_count= 8
control_file_record_keep_time= 0
remote_login_passwordfile= EXCLUSIVE
global_names = TRUE
distributed_transactions = 500
service_names = orcl
instance_name = orcl
mts_dispatchers = (ADDRESS=(PARTIAL=YES)(PROTOCOL=TCP))(DIS=1)(SES=254)(CON=254)(TIC=15)(POO=NO)(MUL=NO)(LIS=(ADDRESS_ LIST=(ADDRESS=(PROTOCOL=IPC)(KEY=PNPKEY))(ADDRESS=(PROTOCOL=TCP)(HOST=127.0.0.1)(PORT=1521))(ADDRESS =(PROTOCOL=IPC)(KEY=orcl))))(PRE=oracle.aurora.server.SGiopServer)
mts_servers = 1
open_links = 4
db_name = orcl
ifile = D:\Oracle\admin\orcl\pfile\init.ora
os_authent_prefix =
job_queue_processes = 2
job_queue_interval = 10
parallel_max_servers = 5
background_dump_dest = D:\Oracle\admin\orcl\bdump
user_dump_dest = D:\Oracle\admin\orcl\udump
max_dump_file_size = 10240
oracle_trace_collection_name=
PMON started with pid=2
DBW0 started with pid=3
LGWR started with pid=4
CKPT started with pid=5
SMON started with pid=6
RECO started with pid=7
SNP0 started with pid=8
SNP1 started with pid=9
Fri Apr 20 15:21:37 2001
starting up 1 shared server(s) ...
starting up 1 dispatcher(s) for network protocol '(ADDRESS=(PARTIAL=YES)(PROTOCOL=TCP))'...
Fri Apr 20 15:21:37 2001
alter database mount
Fri Apr 20 15:21:42 2001
Successful mount of redo thread 1, with mount id 955396998.
Fri Apr 20 15:21:42 2001
Database mounted in Exclusive Mode.
Completed: alter database mount
Fri Apr 20 15:21:42 2001
alter database open
Picked broadcast on commit scheme to generate SCNs
Fri Apr 20 15:21:42 2001
Rolling back half complete log switch of thread 1
***
Corrupt block relative dba: 0x00000075 file=0. blocknum=117.
Bad header found during controlfile block read
Data in bad block - type:255. format:255. rdba:0xffffffff
last change scn:0xffff.ffffffff seq:0xff flg:0xff
consistancy value in tail 0xffffffff
check value in block header: 0xffff, calculated check value: 0x0
spare1:0xff, spare2:0xff, spare2:0xffff
LGWR: terminating instance due to error 227
Instance terminated by LGWR, pid = 153
Fri Apr 20 15:32:52 2001
Starting ORACLE instance (force)
License high water mark = 3