Last yr I obtained a credit report My score was 515-604

Last yr I obtained a credit report. My score was 515-604  I had 2 items showing that I owed on. One was around 170.00 which was mine, and I paid off. The other was a credit card that was not mine(was listed as authorized user). I went through the steps with all 3 Bureaus, to have that removed. One said that my score should go up after paying off, and the removal of the one that wasn’t mine come off. I’m sure  my score went down when it went on there, it should have gone up when it was removed, right? Ok my real concern is this.  I applied for a home loan thinking it would be higher a couple weeks ago, I was denied. I now am showing absolutely no score at all with any of the 3 Burs. I was so upset.     My ? is why? Is it due to inactivity? And when I start building my credit will it start at zero or what. Or will it go back to what it was last yr, and start raising. Is my 600 or so score just gone for good? What is the fastest way to build credit? I think its saying credit cards take a good yr, to show good. What about a say 1,000.00 secured loan say for 6 months. How much would that help? What will help faster and the most cards, or loans? Any help is very much appreciated.

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number one credit mistake in life is thinking if one pays off Bad debts
one will have good credit
 
the number one rule in raising a credit score is to have 3 to 6 rated ( 12 months payment records) line of credit
like credit card and car loans and home loans
 
youve done the right things
its just not enough