It would appear that you choose to update to OS X 10.10.3 - if you then then the Photos App was added to your system which accounts for the different icon - new program, new icon
I(f that is the case then the first time you launch Photos it migrates the last used iPhoto library over
iPHoto is still in your applications folder and still works fine (you need version 9.6.1 - if you were not up to date when you updated the OS see Can't Update iPhoto because it is not available in the App Store
When you get straight on exactly what you have and what program you are running post back with details of what you have and any other info you can provide including messages, errors and details of wha you see
LN