The first thing you should do upon graduating without a job is to retrace your job search steps over the past year. You should immediately recontact all the employers with which you interviewed. If you interviewed at the company-site and failed to make the final cut, you should recontact. If you interviewed on campus and received a form letter rejection, you should recontact. Even if you only went through a short interview on the phone you should recontact.
Why? Several reasons. First, most employers have a minor attrition factor before the actual start date, when some of the accepted offers do not actually start. Better offer. Decided to go back to school. Budding romance in another part of the country. Joined the Peace Corps. Whatever the reason, when there is a dropout before the start date, there is an open position. And when it is this late in the process, few companies want to begin the hiring process all over again. In larger companies there are usually a set number of entry level positions that need to be filled for a full training class. You could be the right person at the right time.
The second reason is that many employers will have made changes in their hiring demand during the intervening weeks or months. If that demand is greater, your notification of immediate availability can make you a prime candidate. Even if there is not an immediate need, any new openings that become available could have your name attached.
The third reason is that your availability has changed. Namely, you are available immediately. Since many medium-sized and smaller companies operate within shorter time frames, they may have shied away from you when your graduation (and availability) was still months away. Your present availability could put you in immediate contention for any currently open positions.
Your new job is out there waiting for you to find it.
The final reason is that even if you come up against a "Sorry, nothing is available" dead end, you have another opportunity to ask for referrals to other companies. Many employers are willing to help you by providing contact information for other employers who may be hiring.
So retrace your steps and notify all past contacts that you are still available. If you are uncomfortable in making this approach, use the excuse of updating the employer with your new address and phone number, vital information if employers are to be able to reach you at a later date. And don't just stop with your employer contacts—recontact your whole network of contacts. There will be a renewed sense of urgency on everyone's part to assist you in your job search. Take advantage of it.