Hi Hari,
I also had faced the same issue once after unfortunate restart of a windows SAP server. The issue was with one of our memory related instance profile parameter. After restart of that server, profile parameter got activated and it became a bottleneck while starting up the instance & DB was not showing in MMC. So, in order to start your server , please perform following tasks synchronously :-
1. Open a command prompt and run "sappfpar check pf = <instance profile location>". Make sure you are running this command from kernel directory to execute sappfpar.
2. If it doesn't show any error , then check if any old backed-up profile is existed in sys/profile directory , if yes then rename it with replace the original one with old one (before that take backup of original instance profile).
3. Now try to start up the system after restarting MMC. If still luck doesn't in favor of you , then update MMC . You can get the MMC from SMP itself.
4. If now it's working fine, then it's well & good, otherwise repair your database with existed database tool (based on your used database).
5. Finally if everything becomes fail to start up your system 
, then restore the old database back-up and try to start the same.
Please let us know how does it go through ??
All the very best ......
Nilutpal.