I have a Roth IRA that was opened when I was single in 2014. I made contributions for the tax year 2014. Now in 2015 I still have that account however I did not make any contributions to it for the year 2015. Now I got married in late 2015 so I want to know if when I file my 2015 taxes, can I file separately or do I have to file jointly?
You seem to have some misconceptions about the tax laws.
Your marital status for Tax Year 2015 is your status as of December 31, 2015.
If you are married as of December 31, 2015, then the decision to file a joint tax return is one that must be made jointly, that is, both spouses must consent to the decision and affirm that decision by signing the joint return. So, you cannot file jointly unless your bride/groom of a few months consents to doing so. If you unilaterally decide that a moral imperative (or tax law) requires you to file a separate tax return, then there is nothing that your spouse can do about it: he/she must perforce file a separate tax return.
US tax law generally wants a married couple to file a joint income tax return (even if only one had income), and encourages this choice by making the rates/rules for the Married Filing Separately (MFS) status onerous. Generally speaking, the sum of the taxes paid on two MFS returns is quite a bit more than the total tax paid on a Married Filing Jointly (MFJ) return; the various cut-offs for eligibility to contribute to IRAs/deduct IRA contributions etc are small, etc. On the other hand, both spouses have joint responsibility for the taxes due on a MFJ return (if one doesn't pay, the IRS will go after the other for the entire amount) as well as the accuracy of the return). If one spouse has reason to believe that the other is cutting corners, not declaring all the income, taking deductions that are not permitted etc (usually all this is on self-employment income, tips, gambling income etc) then the "innocent spouse" might want to decline to file an MFJ return and file MFS instead
There are some special rules that apply if one spouse is not a US citizen and is not a tax resident of the US because of immigration status. Separate tax returns might be required unless the non-citizen spouse chooses to be treated as a US tax resident. This choice will allow the filing of a joint tax return but choosing this option also makes the non-citizen's worldwide income subject to US income tax.
The existence of your previously-opened Roth IRA account (for Tax Year 2014) has no bearing on how you choose to file your 2015 return, even if you opened the Roth IRA account during 2015 and contributed to it by the April 15, 2015 deadline for 2014 contributions. The choice of MFJ versus MFS for 2015 need not be made till April 15, 2016 and could affect the eligibility to contribute to a Roth IRA for 2015 or to deduct contributions to a Traditional IRA because the income limitations for MFJ returns are quite different from income limitations for MFS returns.
You can always file jointly or separately as you like.
I think you are maybe thinking about the income limit for Roth IRA contributions, which for Married Filing Separately the amount you can contribute phases out from $0 to $10000. Since you are not contributing for 2015, this is irrelevant.